The fan case of a turbofan engine directs the axial flow of air in conjunction with the fan during normal engine operation, prevents released fan blades from escaping radially or forwardly and restrains the low pressure shaft radial deflection and blade tips during bird strike events.
The fan is conventionally used in a turbo-fan engine to force a primary air stream through the compressor and turbines of the engine and to force a secondary airflow through an annular radially outward bypass duct. It is essential that the clearance between the rotating fan blades and the internal surface of the fan case be kept within an acceptable range to optimise the fan efficiency. To maintain engine operation and ensure safety, the fan case must also retain or rearwardly deflect released blades, and withstand the effect of bird impact on the blades.
The internal air path surfaces of the fan case are lined with a compressible and a soft abradable material sprayed on the internal fan case surface immediately adjacent the blade tips. During the operation of the engine and rotation of the newly manufactured fan, some of the soft abradable material is removed on contact with the relatively hard tip of the rotating fan blade. A typical thickness for the abradable layer of material is in the order of 0.070 inches. When assembled the tip clearance is in the order of 0.005 to 0.030 inches. During the initial high-speed rotation of the fan, the fan blades stretch elastically under the load of centrifugal force in the order of 0.020 to 0.040 inches. Depending on the heat generated during operation, the blades may thermally expand as well. Due to the dynamic stretching and thermal expansion of the metallic blades, the abradable material is removed on contact with the fan blade tip. Each fan will have its own manufacturing tolerances and the actual degree of running clearance required and stretching of blades will vary a certain amount between different fans when manufactured. The provision of abradable material allows for close tolerance and minimizing of clearance between the fan blade tip and the annular internal air path surface of the fan case.
In the case of small turbofan engines in particular, the clearance between fan blade tips and the fan case internal surface is often of a critical nature. Due to a high aerodynamic loading of the blades, the fan stage stall margin is very sensitive to the tip clearance. Abnormal changes in tip clearance can adversely affect the engine thrust and surge margin.
The fan case and fan must also ensure safe operation of the turbofan engine during two critical conditions; firstly, on the ingestion of birds which strike the fan blades; and secondly, in the event of breakage of a fan blade. These two conditions are known generally as a "bird strike event" and a "blade off event" respectively.
In the prior art, a bird striking the fan generally results in an increase of tip clearance between the fan blade tips and the internal surface of the fan case. The soft abradable material bonded to the interior surface of the fan case is removed together with the compressible material radially outward of the abradable material when the bird strike condition is encountered as follows. When an outboard bird is ingested into the forward fan area, the fan blades cut the bird into fragments and propel the fragments tangentially and axially rearwardly. Depending on the configuration of the flow splitter downstream of the fan, a proportion of the bird fragments are expelled axially through the outward annular by-pass duct, and a portion of bird fragments are ingested into the engine core through the compressor and turbines.
Of particular interest to the present invention is the effect of a bird strike and resulting interaction of the fan blades with the fan case. The fan blades are deformed due to the impact and unbalanced loading. The axial and radial unbalanced loads are transmitted to the low power compressor shaft, the supporting structure and the engine mounts. The fan on the rotating shaft will deflect radially outwardly and cut deeply into the compressible material and abradable material which lines the interior surface of the fan case.
Prior art fan cases for small engines are lined with approximately 0.100 to 0.300 inches of abradable material applied on the interior surface of an approximately 0.300 to 0.500 inch thick layer of compressible material. Twisted and deflected fan blades severely cut into these materials and lead to excessive fan tip clearances.
On a medium bird strike event, regulations require that the engine thrust decreases to no less than 75% of maximum engine thrust within 20 minutes after the bird strike. A number of engine components may be damaged due to the bird strike; however, the cumulative effect of various types of damage cannot reduce the total engine thrust by more than 25%.
Bird strikes may deform the fan blades, damage the engine core, and the compressor blades in addition to increasing the fan blade tip clearance dramatically. It has been found through experiment that excessive fan blade tip clearance can result in 7 to 9% of the thrust loss alone. Considering that regulations require no more than 25% engine thrust loss, it can be seen that excessive fan blade tip clearance after a bird strike is a significant cause of engine thrust loss.
Small diameter fans are extremely sensitive to excessive tip clearance and excessive tip clearance can lead to dangerous stall or surge conditions on encountering "bird strike" events.
The prior art has provided means to limit tip clearance problems on bird strike by providing a hardwall fan case which comprises a stiff fan case shell approximately parallel to the fan blade tips lined with layers of compressible and abradable materials to compensate for manufacturing tolerances and stretch of the blades in operation. Due to excessive movement of the fan blades during a bird strike event, the fan blade tip might wear away the abradable and compressible materials and directly contact the hardwall of the fan case. The fan case is lined with a layer of abradable and compressible materials, since there is a concern that tight clearance during running of the engine will result in dynamic coincidence when the rotor blades rub against the hardwall containment fan case before the rotor stabilizes around its own centre of rotation. The abradable material is therefore used to line a hardwall fan case to give sufficient clearance to stabilize the rotor around its own centre of rotation, without damaging the compressible material during normal running conditions.
A significant disadvantage of a hardwall fan case however, is encountered when a fan blade breaks off in the "blade off" condition. Standard tests are conducted on engine designs wherein a fan blade is released at the maximum permissible engine speed, (known as the red line condition). The fan case structure provides important protection for aircraft and passengers since the rapid rotation of the fan propels the released fan blade tangentially at high speeds. The fan case is provided to contain any released fan blade within the engine itself, or to eject released blade axially rearwardly through the by-pass duct.
A hardwall fan case has a disadvantage resulting from the shape of the internal air path surface. The air path surface generally converges radially inwardly as the air taken into the engine simultaneously increases in pressure and decreases in volume. The internal air path surfaces are tapered radially inwardly such that a released fan blade will bounce off the hardwall fan case and be redirected forwardly. Further catastrophic engine or fuselage damage may occur as a result. The thin sheet metal nacelle in the front of the engine will not contain the released blade propelled with high energy. As a result, regulations require that any released fan blade be directed axially rearwardly to avoid further damage, or be contained within the fan case itself. Deflection of released fan blades forwardly, as well radial expulsion through the fan case itself are very dangerous and unacceptable.
As a result, it has been common to provide a relatively heavy fan case shell, which is lined with compressible material, coated with abradable material. The compressible material acts to absorb the impact of the high velocity released fan blade. However, providing the required thick layer of compressible material shaping the air path surface leads to unacceptable large fan tip clearances during a bird strike event as mentioned above. In the case of relatively large engines however, excessive fan tip clearance is less critical than in small engines.
Therefore, in the prior art there is a conflict between two competing conditions that must be accommodated by fan cases and fan blades. In the case of a medium bird strike, it is preferred that a hardwall fan case be provided to maintain the fan tip clearance within acceptable limits. However, in the case of fan blade breakage, it is preferred to line the fan case with a relatively soft compressible material that can absorb the impact with released fan blade and has a stiff shell surface that can deflect any released fan blade rearwardly.
In the case of a hardwall fan case, the shape of the air pathway tapers inwardly as it progresses rearwardly through the engine and the pressure of air increases with corresponding decrease in volume. By providing a hardwall fan case which follows the air path shape. Generally a released fan blade will be deflected forwardly and impose the risk of unacceptable accidental damage. Released fan blades must be retained within the fan case itself, or be ejected axially rearwardly.
Therefore, it is desirable to provide a fan case structure which can maintain fan tip clearance within acceptable limits after a bird strike event while simultaneously ensuring that any released fan blades are directed axially rearwardly, or retained within the fan case structure itself.
It is also desirable to provide such a fan case structure that will use existing materials and technology without requiring significant rework or re-certification of existing designs.
Further objects of the invention will be apparent from review of the drawings and description of the invention below.